Ex-situ PEM fuel cell testing: towards visualizing gas diffusion

ABSTRACT

The invention comprises novel apparatuses and testing methods for evaluating a fluid diffusion component. The apparatus includes a fluid capable of undergoing oxidation or reduction, a half-cell electrode assembly able to receive the fluid, and a change in acidity indicator in communication with the assembly. An inventive method of the invention includes the steps of passing a fluid capable of undergoing oxidation or reduction through a half-cell electrode assembly to form a sample, contacting the sample with an indicator, and detecting a change in acidity in the indicator.

FIELD OF THE INVENTION

The invention relates generally to fuel cells and particularly tomethods and apparatuses for evaluating individual components of a fuelcell and assemblies of two or more components of the fuel cell.

TECHNICAL BACKGROUND

An ion exchange membrane fuel cell, more specifically a proton exchangemembrane (PEM) fuel cell, produces electricity through the chemicalreaction of hydrogen and oxygen in the air. Within the fuel cell,electrodes, denoted as anode and cathode, surround a polymer electrolyteto form what is generally referred to as a membrane electrode assembly,or MEA. Oftentimes, the electrodes also function as the gas diffusionlayer (“GDL”) of the fuel cell. A catalyst material stimulates hydrogenmolecules to split into hydrogen atoms and then, at the membrane, theatoms each split into a proton and an electron. The electrons areutilized as electrical energy. The protons migrate through theelectrolyte and combine with oxygen and electrons to form water.

A PEM fuel cell includes a membrane electrode assembly sandwichedbetween two graphite flow field plates. Conventionally, the membraneelectrode assembly consists of random-oriented carbon fiber paperelectrodes (anode and cathode) with a thin layer of a catalyst material,particularly platinum or a platinum group metal coated on isotropiccarbon particles, such as lamp black, bonded to either side of a protonexchange membrane disposed between the electrodes. In operation,hydrogen flows through channels in one of the flow field plates to theanode, where the catalyst promotes its separation into hydrogen atomsand thereafter into protons that pass through the membrane and electronsthat flow through an external load. Air flows through the channels inthe other flow field plate to the cathode, where the oxygen in the airis separated into oxygen atoms, which joins with the protons through theproton exchange membrane and the electrons through the circuit, andcombine to form water. Since the membrane is an insulator, the electronstravel through an external circuit in which the electricity is utilized,and join with protons at the cathode. An air stream on the cathode sideis one mechanism by which the water formed by combination of thehydrogen and oxygen is removed. Combinations of such fuel cells are usedin a fuel cell stack to provide the desired voltage.

The flow field plates have a continuous reactant flow channel with aninlet and an outlet. The inlet is connected to a source of fuel in thecase of an anode flow field plate, or a source of oxidant in the case ofa cathode flow field plate. When assembled in a fuel cell stack, eachflow field plate functions as a current collector.

Electrodes, also sometimes referred to as gas diffusion layers, may beformed by providing a graphite sheet as described herein and providingthe sheet with channels, which are preferably smooth-sided, and whichpass between the parallel, opposed surfaces of the flexible graphitesheet and are separated by walls of compressed expandable graphite. Itis the walls of the flexible graphite sheet that actually abut the ionexchange membrane, when the inventive flexible graphite sheet functionsas an electrode in an electrochemical fuel cell.

The channels are formed in the flexible graphite sheet at a plurality oflocations by mechanical impact. Thus, a pattern of channels is formed inthe flexible graphite sheet. That pattern can be devised in order tocontrol, optimize or maximize fluid flow through the channels, asdesired. For instance, the pattern formed in the flexible graphite sheetcan comprise selective placement of the channels, as described, or itcan comprise variations in channel density or channel shape in order to,for instance, equalize fluid pressure along the surface of the electrodewhen in use, as well as for other purposes which would be apparent tothe skilled artisan.

The aforementioned PEM fuel cells are being developed as an alternativeenergy source for portable, stationary, and industrial applications.Significant R&D efforts in the fuel cell area are being directed towardsthe science of fuel cell technology as well as in the areas ofengineering and systems integration. A common need at the heart of allPEM systems is to increase the understanding of molecular levelinteractions within the system including gas flow to the membraneelectrode assembly (“MEA”), diffusion, kinetics, thermodynamics ofreactants and products of the electrochemical reaction, watermanagement, heat transfer, and current collection.

Presently, diagnostic systems like fuel cell test stations are availablewhich allow performance testing of stack-level component integration,combined with electronic measurements for performance evaluations, thesesystems are very costly, complex, and time consuming to operate.Additionally, individual component characterization and materialevaluation is potentially possible through the use of classicelectrochemical, and materials characterization methodologies such asX-ray diffraction, Potentiostatic/Galvanostatic measurements, impedanceanalysis, and microscopy.

As an example of industry shortcomings in the testing regime, Gurleyporosity is commonly utilized to give an indication of the permeabilityof a fuel gas (e.g., hydrogen) through gas diffusion layer substrates.While Gurley porosity is useful for initial material screening purposes,direct correlation to operational performance is difficult. Also Gurleyporosity does not include any specificity towards a correlation with theelectrochemical reaction that takes place at the anode or cathode.Furthermore, localized differences in gas diffusion rates are difficultto detect.

There is a lack of availability, of intermediate testing paradigms thatelucidate material and component integration, below the stack-level oreven single cell level integration (ex-situ). Also there is need fortesting methods to evaluate component performance functions underconditions that simulate real fuel cell operation. Furthermore, there isa need for a quick cost-effective testing paradigm for components.

SUMMARY OF THE INVENTION

One aspect of the invention is a fluid diffusion testing apparatus. Theapparatus includes a fluid capable of undergoing oxidation or reduction.The apparatus further includes a half-cell electrode assembly capable toreceive the fluid. Additionally, the apparatus includes a change inacidity indicator in communication with the assembly.

Another aspect of the invention is a method of testing a fluid diffusionassembly. The method includes the step of passing a fluid capable ofundergoing oxidation or reduction through a half-cell electrode assemblyto form a sample. The sample contacts an indicator. A change in acidityof the indicator is detected.

A further aspect of the invention is a method for selecting a fluidpermeable element for a proton exchange membrane fuel cell. The methodincludes the steps of (a) passing a fluid capable of undergoingoxidation or reduction through a half-cell electrode assembly to form asample; (b) contacting the sample with an indicator; (c) observing atleast one concentration gradient of the sample in the indicator; (e)conducting the above steps (a)-(d) on a plurality of half-cell electrodeassemblies; and (f) selecting the half-cell electrode from the pluralitywith the most uniform concentration of the sample in the indicator.

An additional aspect of the invention is a method of visualizing gasdiffusion. The method of visualizing gas diffusion includes the step ofpassing a fluid capable of undergoing oxidation or reduction through ahalf-cell electrode assembly to form a sample. The method furtherincludes the step of the sample contacting an indicator. Another step ofthe method is observing at least one concentration gradient of thesample in the indicator.

Furthermore, aspects of the invention include a method of measuring gasdiffusion. The method of measuring gas diffusion also includes the stepsof passing a fluid capable of undergoing oxidation or reduction througha half-cell electrode assembly to form a sample and contacting thesample with an indicator. The method of measuring gas diffusion furtherincludes the step of determining a difference in the pH of a mixture ofthe indicator and the sample as compared to the pH of the indicatorsubstantially free of the sample.

One advantage of the invention includes the ability to visualize the gasdiffusion activity in PEM half-cell reactions. In an embodiment of theinvention, this is accomplished via a novel integration of fluorescentpH detection or a colorimetric dye with materials that may be used in aPEM fuel cell.

Another advantage of the invention is that the invention can be used tofocus on parameters related to spatial visualization of gas diffusion,catalyst uniformity verification, and fuel delivery through gasdiffusion layer(s) to the catalyst.

A further advantage of the invention is that it may be practiced to testvarious components of a fuel cell, individually or in combination witheach other, at conditions that truly resemble the operating conditionsof the cell.

Furthermore, the advantages include that the invention may be used toevaluate differences in gas diffusion properties between materials whosefunction is to diffuse a fluid. This may be accomplished byvisualization of active surface sites and the spatial resolution of thesites for the material, which represents the anodic dissociation ofhydrogen (in one case) in the separation of protons and electrons (orthe consumption of protons in the case of the cathode reaction).

Additionally, the invention will accelerate the state of the artunderstanding of component functions within an operating fuel cell, andprovide a tool to assist in rapidly commercializing the fuel cellthrough optimized component integration. The invention also providestesting paradigms for sub-stack and sub-cell material assembly andcomponent integration. Furthermore, the inventive apparatus can be usedto obtain ex-situ diagnostic outputs with respect to componentfunctionality.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, the claims, as well as theappended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate various embodiments of theinvention, and together with the description serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side schematic representation of a fluid diffusion testingapparatus.

FIG. 2 is a top view of an indicator having at least one area of changedacidity.

FIG. 3 shows image capture in dark conditions to directly visualize thefluorescence from the active areas of hydrogen conversion to protons.

FIG. 4 is a digitally filtered image to allow improved contrast anddetection of the blue fluorescence.

FIG. 5 is a schematic side elevation view of the alignment of ahalf-cell electrode assembly 16, and an indicator 18 in accordance withthe invention.

FIG. 6 is a top view of indicator 18 depicted in FIG. 5.

FIG. 7 is a top view of a concentration gradient 22 of sample associatedwith at least a portion of a catalyst in a solution of indicator 18 andan oxidizable fluid (e.g. methanol (MeOH)).

FIGS. 8 and 9 comprise two top views of indicator 18 which each includea respective concentration gradient of sample in indicator 18.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be further described in regards to the accompanyingdrawings. Whenever possible, like or the same reference numerals may beused to describe like or the same elements. Illustrated in FIG. 1 is aschematic representation of a fluid diffusion testing apparatus,generally designated 10. The embodiment of the apparatus 10 illustratedin FIG. 1 is an electrochemical cell allowing the anode half-cellreaction of preferably, but not limited to, H₂2H⁺+2e⁻. In the samemanner the half-cell reaction of other proton generating fuels can beutilized for any PEM reaction, such as methanol (MeOH). Theaforementioned reaction may also be known as the anode reaction or anoxidation reaction. The invention is not limited to the aforementionedreaction. The invention is equally applicable to a cathode half-cellreaction, such as, but not limited to, 2H⁺+O⁻²H₂O. The cathode half-cellreaction may also be known as a reduction reaction or cathode reaction.

Apparatus 10 includes a fluid capable of undergoing oxidation orreduction. One example of a fluid able to undergo an oxidation reactioncomprises hydrogen. An example of a fluid capable of undergoingreduction comprises a proton such as H⁺. Preferably, the hydrogen gaswas generated from an electrolyzer, not shown in FIG. 1. Other sourcesof the fluid may also be used. For example, if the fluid comprisesoxygen, potential sources include air and bottled oxygen. Additionally,humidity could be a potential source for oxygen or hydrogen. Fluid isused herein to mean a composition of matter that is either in the gasphase, liquid phase, or some combination of both. As shown in FIG. 1,the fluid may be passed through a tube 12. Preferably tube 12 includes aflange 14.

A half-cell electrode assembly 16 capable to receive the fluid isaligned to receive the fluid in fluid 14. Preferably assembly 16 is ableto generate a proton or able to reduce a proton, as described above.Furthermore, the aforementioned half-cell oxidation or reductionreaction takes place at assembly 16. Preferably, assembly 16 includes acatalyst for one of the aforementioned oxidation or reduction reactions.The catalyst may also comprise an ink.

Examples of suitable catalysts comprise transition metals, preferablynoble metals, such as platinum, gold, silver, palladium, ruthenium,rhodium, osmium, and iridium, and combinations thereof. A preferredcatalyst is platinum black on carbon, or platinum/ruthenium on carbon.

Optionally, assembly 16 may also include a fluid permeable element. Thefluid permeable element may be an integral part of assembly 16 or,alternatively, adjacent the catalyst. It is preferred that the fluidpermeable member and the catalyst are in communication, meaning that thefluid can be passed from the fluid permeable member to the catalyst.Preferably, the fluid permeable element comprises at least one of a gasdiffusion layer, a gas diffusion substrate, flow field plate, andcombinations thereof. Preferably, the gas diffusion substrate comprisesat least one of a sheet of flexible graphite, a carbon fiber paper, acomposite of flexible graphite and a polymer, a composite of carbon anda polymer, and a composite of flexible graphite, carbon, and a polymer.Other suitable diffusion substrates comprise cotton fiber, glass fiber,polymer asbestos, cardboard, aluminum oxide membrane, zeolite substrate,and inorganic fiber (e.g., mullite). Examples of suitable polymersinclude phenolic resins, acrylic resins, and epoxy resins. Optionally,the polymer may be in the form of a fiber or a perforated sheet. Oneexample of flexible graphite is GRAFCELL™, from Graftech Inc. ofLakewood, Ohio. Optionally, the sheet of flexible graphite may have atleast one perforation, preferably a plurality of perforations.Preferably, the perforation is aligned in communication with thecatalyst. Optionally, the gas diffusion layer may comprise a carboncoating, a carbon black coating, a polytetrafluoroethylene coating ormixtures thereof.

In addition to the catalyst and the optional fluid permeable element,apparatus 10 may also include a membrane (also referred to as anelectrolyte). Preferably, the membrane comprises a solid polymerelectrolyte (also referred to as a solid polymer ion exchange membrane)that is an electrically insulating material. More preferred theinsulating material is substantially gas-impermeable and substantiallyion-permeable. Suitable solid polymer materials include films ofperfluorinated sulfonic acids derived from fluorinated styrenes,quaternary amine polystyrene, polybenzimidazole (“PBI”), or otherionomeric polymers.

As for properties, it is preferred that the membrane has excellentmechanical strength, predictable dimensional changes, high electricalconductivity, and the ability to transport the desired ions whilerejecting the undesired ions and molecules.

With respect to the membrane, examples of suitable membrane materialsinclude, but are not necessarily limited to, NAFION® products availablefrom Dupont of Wilmington, Del., the Dow membrane materials availablefrom Dow Chemical Co., of Midland, Mich., the Gore-Select™ materialsavailable from W. L. Gore & Associates, Inc, of Wilmington, Del. In analternate embodiment of assembly 16, the membrane may be an integralpart of the catalyst or separate and apart from the catalyst.

For additional background regarding the catalyst, the fluid permeableelement, the membrane, or other basic elements of an electrochemicalfuel cell, the specifications of U.S. Pat. Nos. 4,988,583 and 5,300,370and PCT WO 95/16287 are incorporated herein by reference in theirentirety.

Optionally, in the case of the oxidation reaction, it is preferred thatassembly 16 performs the function of removing the electron (e⁻) fromapparatus 10. One technique to remove the electron from apparatus 10 isto ground apparatus 10.

Preferably, apparatus 10 also includes a change in acidity indicator 18.It is preferred that indicator 18 is in communication with assembly 16.Communication is used herein to mean that matter may flow into assembly16 and at least the desired material (e.g., proton, electron, or reducedspecie) may pass into indicator 18, or that indicator 18 may detect thegeneration of the desired material by some other means. Preferably,indicator 18 receives the desired material from assembly 16. The desiredmaterial may also be referred to as the product of the reaction or the“sample.” The sample is used herein to describe the proton generated orthe compound reduced depending on whether the half-cell reaction is acathode reaction or an anode reaction. Indicator 18 does not require anaqueous medium or any other sort of aqueous environment.

In one embodiment, indicator 18 comprises a fluorescent pH indicator andan UV lamp aligned to illuminate the fluorescent pH indicator.Optionally, the fluorescent pH indicator may comprise a solution in theform of a liquid or a gel. Preferably, the gel may comprise ahydrophilic material, more preferably a hydrophilic material that swellsin the presence of water. Preferred types of the fluorescent pHindicators include any element or matter that is able to detect a pKarange of acidity that is generated by the half-cell reaction such as apH sensitive dye. Sources of pH sensitive dyes include Aldrich of St.Louis, Mo. or Molecular Probes Inc. of Eugene, Oreg. One such pHsensitive dye comprises quinine. Examples of other types of fluorescentpH indicators include Eosin B, Eosin Y, and Fluorescein. Eosin B andEosin Y both comprise disodium salts. A preferred type of UV lamp is ablack lamp and a preferred range of wavelengths comprises at least about250 nm and no more than about 400 nm. One source of a suitable UV lampis Fisher Scientific of Springfield, N.J.

Another example of a suitable indicator 18 would be a colorimetric dye.Preferably, the colorimetric dye will change color upon transfer of thesample (proton, electron, or a reduced specie) to indicator 18. Forexample, upon the aforementioned transfer of the sample, thecolorimetric dye may change colors from clear to a particular color(e.g., red or green) or vice versa. In another embodiment, thecolorimetric dye may change from one color to another such as from redto green upon the transfer. Phloxine B from Aldrich Chemical Co. is oneexample of a suitable colorimetric dye. In a generic sense, Phloxine Bcomprises spiro[isobenzofuran-1 (3H), 9′-[9H]xanthen-3-one, 2′ 4′ 5′7′-tetrabromo-4,5,6,7-tetrachlor-3′ 6′-dihydroxy-,disodium salt.

Preferably, the colorimetric dye does not require the presence of a UVlamp to observe the aforementioned color change. Preferably, thelighting available is ambient or room light available from any type ofcommon household light bulb or sunlight. Therefore, an advantage tousing a colorimetric dye as an indicator is that a UV lamp would not beneeded.

In a third embodiment, indicator 18 may comprise a potentiometric dye tomeasure a current generated during the reaction that takes place at thecatalyst, with or without a colorimetric dye or a fluorescent pHindicator. Also various types of hardware that may be used to detect achange in indicator 18 include a fluorescent microscope with or withouta band pass filter, or an imaging microscope wavelength light detectorsuch as a Near-field Scanning Optical Microscopy (NSOM). An advantage ofthe hardware is that the hardware is able to detect a change inindicator 18 with higher resolution than that of the human eye.

In specific embodiments, indicator 18 may comprise a solution of a pHindicator and a host. Examples of suitable hosts includes water, Nafion®(a perfluorinated sulfonic acid), an organic solvent, or a catalystsupport. The host may act as a binder to adhere indicator 18 to assembly16. The host may maintain indicator 18 in contact with the catalyst.Optionally, the solution may include at least one stabilizer, at leastone viscosity enhancer and/or at least one component able to transfercurrent. The viscosity enhancer may be used to control the viscosity ofindicator 18 such that indicator 18 have a viscosity associated with agel or a liquid. An example of a suitable stabilizer is ethylenediamnetetraacetic acid (“EDTA”). Other examples of suitable viscosityenhancers include glycerin, gel, oil, guar gum, methyl cellulose, andhydrous magnesium clay, e.g., Laponite®. Examples of components able totransfer current include NaCl, NaClO₄, Na PF₆, NaBF₄, and mixturesthereof.

In practice, it is possible to apply a two or three terminalelectrochemical diagnostic system such as a standard potentiostat,galvanostat, voltmeter, or a preferably sensitive ammeter (such as apicoammeter) for electrode testing, determining the amount of electronsgenerated, and/or current collection. Preferably, the ammeter is able tomeasure picoamps to milliamps of current.

The invention may also include a method of testing a fluid diffusionassembly. The method includes passing the fluid through half-cellelectrode assembly 16 to form a sample. The sample is contacted withindicator 18. Diffusion is an example of one type of fluid flowmechanism that may be used to contact the sample with indicator 18. Achange in acidity in indicator 18 is detected. In one embodiment,detecting comprises a change in the acidity of indicator 18 due to thepresence of at least one proton in indicator 18. Optionally, the step ofdetecting may comprise illuminating the fluorescent pH indicator withthe UV lamp. Also, this method may be used to test the diffusion of thesample from assembly 16 in one or more of the three geometricaldimensions relative to assembly 16.

The method may also include the step of observing at least oneconcentration gradient of the sample in indicator 18. This isillustrated in FIG. 2, generally designated 20. Depicted in FIG. 2 is atop view of indicator 18 having areas of change in acidity representedby circled concentration gradients 22 of protons in indicator 18.Gradient 22 may be in one or more of the three geometrical dimensionsrelative to assembly 16.

As depicted in FIG. 2, blue fluorescence (gradients 22) (from a quinineindicator 18 with no proton generating species or solvent) is seen inactive areas along the surface of indicator 18. In FIG. 2, electrodes 24in the solution were not applying a bias on the sample. However, FIG. 2does demonstrate how alternate electrochemical diagnostics can beincorporated into the invention, such as, open circuit potential vs.time or cyclic voltammetry. One advantage of having electrodes 24 inindicator 18 is that the electrochemical response during operation maybe measured, such as by measuring impedance or current.

The method may further include the step of altering the design ofassembly 16 based on at least one of the results of the observing step.A non-exhaustive list of changes to assembly 16 includes changes to thegas diffusion layer, changes to the gas diffusion substrate, and changesto the gas delivery system (e.g., flow field plate). Examples of changesin the gas diffusion layer and gas diffusion substrate include changesin the choice of materials, pattern of openings in either the layer orthe substrate, the sizes of the holes or porosity in either the layer orthe substrate, and composition of either the layer or the substrate.Changes to the gas delivery system may include the design of thechannels in the flow field plate, changes in the composition of the flowfield plate, or changes in the thickness of the flow field plate.

The invention further includes a method for selecting a fluid permeableelement for a proton exchange membrane fuel cell. The method includesthe step of flowing the fluid through half-cell electrode assembly 16 toform a sample. The sample contacts indicator 18. A change in acidity inindicator 18 is detected. Preferably at least one concentration gradient22 of the sample is observed in indicator 18. Preferably, gradient 22 ison a top surface of indicator 18. Preferably, the aforementioned stepsregarding the method of selecting are conducted on a plurality of thehalf-cell electrode assemblies 16. The half-cell electrode assembly 16from the plurality with the most uniform concentration of the sample, inindicator 18 is selected. In the case of multiple assemblies 16 withuniform gradients 22, the assembly 16 with the gradient 22 that has thehighest color intensity is selected. Techniques to judge intensityinclude visualization or the below noted spectroscopy and digital imagecapture techniques. This method may also be used to determine apreferred fluid delivery system for a fuel cell, which multiple fluiddelivery systems are proposed.

An example of what is meant by “most uniform concentration of the samplein indicator 18” is shown in FIGS. 8 and 9. FIGS. 8 and 9 are top viewsof indicator 18 for two different assemblies 16. The dark areas in eachof FIGS. 8 and 9 represent concentration gradients 22. As depicted inFIG. 8, concentration gradients 22 has a pattern across a top surface ofindicator 18 resulting from the testing of the assembly 16 for FIG. 8.In contrast, testing of assembly 16 for FIG. 9 resulted in four (4)random locations of gradients 22 on a top surface of indicator 18. Thus,the assembly, which exhibited the most uniform concentration of thesample in indicator 18 between FIGS. 8 and 9, was FIG. 8. Therefore, inselecting an assembly 16, the person of ordinary skill in the art shouldselect the assembly associated with FIG. 8. Optionally, the method mayfurther comprise forming the half-cell electrode assembly 16 with themost uniform concentration of the sample in indicator 18 into a fuelcell.

Furthermore, the invention includes a method of visualizing gasdiffusion. The method includes the step of passing the fluid throughassembly 16 to form the sample. The sample contacts indicator 18 and anychange in acidity in indicator 18 is detected. Indicator 18 is observedfor any concentration gradients 22 in indicator 18. The observing stepmay comprise viewing at least one colored area in indicator 18.Optionally, image capture, magnification, or spectroscopic tools may aidthe observing step.

The method may also include the step of altering a design of half-cellelectrode assembly 16 based on at least one result of the observingstep. Another step that may be included into the method is the step ofincorporating half-cell electrode assembly 16 into a fuel cell based onat least one result of the observing step.

This aspect of the invention may also be used to determine the flow ofthe fluid through the fluid permeable element of assembly 16 as comparedto flow of the fluid through the catalyst. One technique of how this isaccomplished is by comparing FIGS. 2 and 7.

FIG. 7 is a top view of a concentration gradient 22 associated with atleast a portion, preferably the entire body, of catalyst of an assembly16 submerged in a solution of indicator 18 and a oxidizable fluid (e.g.methanol (MeOH)), generally designated 70. As depicted in FIG. 7, thepresence of gradient 22 in the solution resulted in the half-cellreaction of the fluid in the solution, in the vicinity of the catalyst.This is apparent by the uniform concentration gradient 22 in thesolution. This discloses to a person of ordinary skill in the art thatthe catalyst is uniformly coated onto the support material submerged inthe solution.

A person of ordinary skill in the art could compare FIG. 7 to FIG. 2,which includes the seven (7) gradients 22. The randomness of gradients22 in FIG. 2 would indicate to a person of ordinary skill in the artthat fluid is not uniformly being flown through assembly 16 and intocontact with the catalyst.

Therefore, when assembly 16 comprises a catalyst layer the method mayinclude the steps of submerging the catalyst layer in a solution ofindicator 18 and the fluid, detecting a change in acidity in thesolution, and comparing a concentration gradient of change in theacidity of the solution to the concentration gradient of the sample inindicator 18. Also, the aforementioned step of submerging the catalystin the solution may be used to analyze the uniformity of the catalystused in assembly 16.

Another aspect of the invention includes a method of measuring gasdiffusion. The steps of the method include the following: (a) passingthe fluid through half-cell electrode assembly 16 to form the sample;(b) contacting the sample with indicator 18; and (c) determining adifference in the pH of a mixture of indicator 18 and the sample ascompared to the pH of a solution of indicator 18 substantially free ofthe sample.

The method may also include the step of determining a concentration ofacidic matter in the mixture. Another optional step may be determiningan amount of electrons generated as a result of the passing step.Examples of suitable equipment, which may be used to determine theamount of electrons generated, include potentiostats, voltmeters, orammeters.

The spatial resolution of active surface regions (also referred to asgradients 22) of indicator 18 demonstrates an aspect of the usefulnessof this invention. The spatial resolution of gradients 22 in indicator18 can be used to develop an understanding of how material modificationscan affect gas diffusion, catalyst efficiency, proton diffusion, and theinterplay between gas diffusion, catalyst efficiency and protondiffusion.

The invention may also be used to evaluate material property differencesas can be seen upon comparison of material types such as, for example,carbon fiber paper versus flexible graphite sheets. The invention may beused to predict morphology differences, such as micro porosity and poresize distribution, and to evaluate differences in the diffusion of gasand subsequent delivery to the catalyst layer.

Many aspects of PEM fuel cell materials, components, and/or operationalparameters can be evaluated with this invention, such as:

-   -   a) Spatial resolution of gas diffusion        -   a. Effects due to operating parameters such as temperature,            fuel to air stoichiometry, voltage, current, fuel, or air            humidity,    -   b) Gas Diffusion Substrate morphology changes, and/or material        changes,    -   c) Catalyst uniformity, deposition procedure, composition, or        ink formulation,    -   d) Diagnostic outputs for anode or cathode half reaction:        -   a. Current collection/generation,        -   b. Reactant/byproduct mass transfer, and        -   c. Electrode testing and efficiency—i.e., polarization            curves.    -   e) Various indicator solutions, gels or alternate medium can be        evaluated, as well as additives to the same. Alternatively these        can be applied to the catalyst ink for direct detection within        that layer. The same principle can be applied to the proton        conduction membrane or material.    -   f) PEM membrane evaluations:        -   a. Thickness, proton diffusion rates        -   b. Membrane acidity        -   c. Membrane hydration effects        -   d. Membrane composition effects        -   e. Membrane manufacture,    -   g) Flow field & MEA designs related to gas delivery to the MEA,    -   h) Application to alkaline cells and other industrially relevant        electrochemical reactions,    -   i) Improved spatial resolution via Near-field Scanning Optical        Microscopy and Fluorescent Imaging techniques, among other        imaging or spectroscopic techniques,    -   j) PEM fuel cell applications—not just hydrogen but alcohol        (direct methanol, etc.), acetic acid, formic acid, etc., any PEM        reaction, and    -   k) Intrinsic material property changes during fuel cell        operation such as electrical conductivity of the GDL substrate

Considering the anode half-cell reaction in a hydrogen fuel cell(H₂→2H⁺+2e⁻), it can be imagined that where active catalyst sitesconvert hydrogen gas into protons, there will be a localized region ofhigher acidity providing cage escape of the electron to precluderecombination with the protons. Preventing recombination of the electronwith the proton can optionally be achieved by simply grounding thesubstrate in a closed circuit fashion (with or without external bias).Utilizing a common fluorescent pH indicator, a test cell for visualizingthis half-cell reaction can be constructed. In a similar manner, thehalf-cell reaction of other proton generating fuels can be utilized forany PEM reaction. An example has been demonstrated for purposes ofscreening optimal catalyst compositions in methanol reactions. Aconverse approach can be taken to construct a test cell for the cathodehalf reaction, utilizing the consumption of protons as the mechanism forreaction detection, visualization, and measurement. In addition, thisinvention is intended to describe at least a method to evaluate thefollowing without being limited to the below applications:

-   -   1) Gas diffusion layers consisting of various formulations,        e.g., diffusion layers composed of a polymer and carbon and/or        graphite    -   2) Proton exchange materials and/or membranes alone, and proton        diffusion through the same    -   3) Substrates for the above—such as but not limited to gas        diffusion substrates, membrane electrode assembly components        (i.e., carbon fiber paper, fibrous materials, carbon or        graphite-based materials, flexible graphite, etc.—with or        without incorporation of the proton exchange material    -   4) MEA assemblies including any or all of the components for        comparison such as flow-field plates, gas diffusion substrates,        gas diffusion coatings, layers, membranes, catalyst support        formulations, catalyst materials, and additives,    -   5) Flow channel designs,    -   6) Operational parameters such as fuel and/or oxidant gas        pressure, flow rate, and composition,    -   7) Design and/or performance of gas diffusion substrate        morphology or composition (such as fiber papers or perforated        GRAFOIL® flexible graphite materials),    -   8) Design and/or performance of fibrous material morphology or        composition (such as but not limited to carbon fiber papers),    -   9) Design and/or performance of conductive or non-conductive        composite materials morphology or composition.    -   10) Anode or cathode engineering designs,    -   11) Electron generation or consumption can be spatially        “mapped”, and    -   12) Mass transport of water may be evaluated.

For example, use of the following technique will allow varioussubstrates to be compared. This application of the invention has beenverified with an embodiment as shown in FIG. 1 where hydrogen gas wasfed into the bottom of apparatus 10. This specific example shown shouldnot limit the general application of this invention for “mapping” theelectrochemical activity. Mapping is used herein to mean at least theactivity of observing gradients 22 in indicator 18 and the documentationand retention of where gradients 22 appeared in indicator 18 withrespect to a particular assembly 16.

Digital image capture can be utilized as shown in FIGS. 3 and 4 formapping purposes to visualize active areas of the surface of indicator18. FIG. 3 shows image capture in dark conditions to visualize thefluorescence from the active areas (gradients 22) of hydrogen conversionto protons (or alternatively consumption of protons). In addition tomapping the location of areas 22, the intensity of areas 22 may bemapped. FIG. 4 is a digitally filtered image to allow improved contrastand detection of the blue fluorescence (gradients 22). Again, FIGS. 3and 4 were acquired with no externally applied potential bias on thesample or indicator 18. Optional steps to enhance image resolution mayinclude optical filtering techniques. For example, the data acquired asin FIGS. 3 and 4 above could be enhanced with the aid of CCD cameraacquisition fed through narrow band pass filters, optical fluorescencemicroscopy, spectrometer analysis, and/or additional optical filters forhardware enhancements. These alterations may lead to better imageresolution, definitive spectral analysis, and morphology correlations.

The images and visualizations such as shown in FIGS. 2, 3, and 4 will bebeneficial for at least engineering a gas diffusion substrate materialwhich is optimized and has uniform gas diffusion resulting in uniformcatalyst utilization.

The delivery of gas through flow field channels, through the diffusionsubstrate, and reaching the catalyst can be visualized with thisinvention. In practice mathematical modeling can be used to predictbeneficial results and to create beneficial designs for flow-fieldchannels. The invention can be used to verify the models and/or torefine the models, thereby leading to better engineering designs.Uniformity of gas delivery to the catalyst can be visualized as abovewith this invention wherein discrete, localized areas (or localizedincreased & decreased areas) of proton generation follow the design ofthe flow channels.

This application is further illustrated in FIGS. 5 and 6. FIG. 5 is aschematic side elevation view of the alignment of a half-cell electrodeassembly 16, and indicator 18. Half-cell electrode assembly 16 comprisesof a flow field plate 52 with channels (not shown) and a layer 54, whichcomprises a gas diffusion substrate, gas diffusion layer coating, and acatalyst. Preferably, indicator 18 comprises a material able to indicatethe presence of protons through fluorescence.

FIG. 6 is a top view of a surface of indicator 18 shown in FIG. 5. Asdepicted in FIG. 6, indicator 18 includes a plurality of fluorescentareas 62. Each fluorescent area 62 represents an area of changed acidityin indicator 18 due to the presence of at least one proton. FIG. 6indicates that the fluid is being transmitted through assembly 16 anduniformly contacting the catalyst in linear regions 64 of fluorescentareas 62. Additionally, cylindrical fuel cell designs and materials canbe analyzed in a similar fashion.

The invention will be further described with the foregoing examples. Theexamples only include various embodiments of the invention and are notmeant to limit the invention.

EXAMPLES Example 1

Test Apparatus

A ½ MEA sample (assembly 16) was prepared with a perforated flexiblegraphite gas diffusion substrate (available from Graftech Inc. ofLakewood, Ohio as GRAFCELL™) with a high surface area carbon gasdiffusion layer having a carbon layer (the carbon was from Cabot). A 20%Pt-carbon black catalyst was blade coated onto the gas diffusion layer.The MEA was at least about 4 thousands of an inch thick.

The MEA was placed into flange 14 of apparatus 10, schematically shownin FIG. 1. Above the MEA was indicator 18. Indicator 18 was a liquidmedium composed of a dye/indicator solution with a variety ofconstituents that highlights subsequent dissociation of protons from theelectrochemical reaction. The solution consisted of an electrolyte,fluorescent pH indicator with appropriate pKa for the reaction medium,stabilizer, and viscosity additive. Indicator 18 included about 50 ml ofwater, about 100 ml-molar concentration of quinine, about 1 molarconcentration of an electrolyte of sodium chloride and sodiumtetrafluoroborate, and up to about 90-wt % of glycerin, preferably up toabout 10%. A preferred viscosity of indicator 18 was about 50 secondswhen measured with a #2 Zahn cup. A UV lamp was shown onto the sampleand the solution which made apparent the excitation of the fluorescentpH indicator when the pH was below the indicator pKa value for quinine.

Results and Discussion

An anode gas (hydrogen) was allowed to pass through the thickness of theMEA. Upon reaching the catalyst surface layer, hydrogen was dissociatedinto protons and electrons. The electrons were collected through theplane of the gas diffusion substrate and removed from the system toprevent recombination with the protons. The protons diffused through thecatalyst layer and into indicator 18 residing above the top surface ofthe MEA. The diffusion of protons throughout the solution allowedadequate spatial resolution of the blue fluorescence from the indicatordye as shown in FIG. 2. As shown in FIG. 2, the circled areas (gradients22) indicate regions of active hydrogen dissociation into protons fromthe anode.

Example 2

Example 2 is an example of verification that relatively uniform catalystmaterial is being deposited is stated below. In this example, the fluidwas methanol and reaction was the oxidation reaction resulting inhydrogen dissociating from the methanol. The gas diffusion layer (“GDL”)sample comprised of a gas diffusion substrate, GDL coatings, andcatalyst layer was immersed in a methanolic solution of dye indicator18. An anodic potential was applied to the GDL/catalyst material andprotons generated from the methanol oxidation reaction were visualizedvia fluorescence.

Results and Discussion

The surface of the indicator showed quite uniform fluorescence. Thiswould indicate that the layer had a uniform covering of catalyst andthat the catalyst material was still active. Therefore, any non-uniformproton generation from the half-cell apparatus images (as discussedabove) is not due to non-uniform catalyst deposition, or poisoned(inactive) catalyst spots on the material.

Alternate embodiments of the invention can be utilized by systematicallyonly varying one variable at a time, such as the gas diffusion substratetype, GDL coating type, composition, or morphology. Comparisons can bemade that isolate a variable and allow ex-situ testing of that variableor component aside from testing in a fuel cell test station.Furthermore, other materials and/or variables can be isolated and testedin a similar fashion.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus it isintended that the present invention cover the modifications andvariations of the invention provided they come within the scope of theappended claims and their equivalents.

1. A method of testing a fluid diffusion assembly comprising: passing afluid capable of undergoing oxidation or reduction through a half-cellelectrode assembly to form a sample; contacting said sample with anindicator; observing at least one concentration gradient of said samplein said indicator; detecting a change in acidity in said indicator; andaltering a design of said half-cell electrode assembly based on at leastone result of said observing step.
 2. The method of testing a fluiddiffusion assembly according to claim 1 wherein said indicator comprisesone of a colorimetric dye, potentiometric dye, or a fluorescent pHindicator.
 3. A method for selecting a fluid permeable element for aproton exchange membrane fuel cell comprising: (a) passing a fluidcapable of undergoing oxidation or reduction through a half-cellelectrode assembly to form a sample; (b) contacting said sample with anindicator; (c) detecting a change in acidity in said indicator; (d)observing at least one concentration gradient of said sample in saidindicator; (e) conducting the above steps (a)-(d) on a plurality ofhalf-cell electrode assemblies; and (f) selecting the half-cellelectrode assembly from said plurality with the most uniformconcentration of said sample in said indicator.
 4. The method forselecting a fluid permeable element of claim 3 wherein said detectingcomprises illuminating a fluorescent pH indicator with a UV lamp.
 5. Themethod for selecting a fluid permeable element according to claim 3further comprising forming said half-cell electrode assembly from saidplurality with the most uniform conventration of said sample in saidindicator into a proton exchange membrane fuel cell.
 6. The method forselecting a fluid permeable element according to claim 3 furthercomprising detecting an amount of electrical current generated duringsaid passing.
 7. The method for selecting a fluid permeable elementaccording to claim 3 wherein said observing comprises seeing a change incolor in said indicator, wherein said indicator comprises a colorimetricdye.
 8. A method of visualizing gas diffusion comprising: passing afluid capable of undergoing oxidation or reduction through a half-cellelectrode assembly to form a sample; contacting said sample with anindicator; observing at least one concentration gradient of said samplein said indicator; and altering a design of said half-cell electrodeassembly based on at least one result of said observing step.
 9. Themethod of visualizing gas diffusion according to claim 8 furthercomprising incorporating said half-cell electrode into a fuel cell basedon at least one result of said observing step.
 10. The method ofvisualizing gas diffusion according to claim 8 wherein said assemblycomprises a catalyst layer and further comprising submerging saidcatalyst layer in a solution of said indicator and said fluid, detectinga change in acidity in said solution, and comparing a concentrationgradient of change in the acidity of said solution to said concentrationgradient of said sample in said indicator.
 11. A method of visualizinggas diffusion comprising: passing a fluid capable of undergoingoxidation or reduction through a half-cell electrode assembly to form asample; contacting said sample with an indicator; observing at least oneconcentration gradient of said sample in said indicator; andincorporating said half-cell electrode into a fuel cell based on atleast one result of said observing step.
 12. The method of visualizinggas diffusion according to claim 11 further comprising altering a designof said half-cell electrode assembly based on at least one result ofsaid observing step.
 13. The method of visualizing gas diffusionaccording to claim 11 wherein said assembly comprises a catalyst layerand further comprising submerging said catalyst layer in a solution ofsaid indicator and said fluid, detecting a change in acidity in saidsolution, and comparing a concentration gradient of change in theacidity of said solution to said concentration gradient of said samplein said indicator.